Development of information and communications technology has been remarkable in recent years, and demand for further improvement in speed and advancement of the technology has been expanding. In connection with this, quantum information processing is known as art different from conventional information processing which deals with electric signals. Research on the quantum information processing, which was ignited by a quantum encryption protocol devised by Charles Bennet and others in 1984 (IBM) and a quantum computing algorithm devised by Peter Shore and others in 1994 (AT&T), has now been energetically advanced.
According to the quantum encryption, safety is guaranteed by physical development, in view of the Heisenberg uncertainty principle in quantum mechanics. According to the Heisenberg uncertainty principle, a state changes with monitoring; therefore, when communication is intercepted (monitored), the interception always becomes obvious. Actions such as blocking of communication can be taken in response to the interception. As such, the interception of communication is believed to be physically impossible. Furthermore, it is also impossible to reproduce particles due to the Heisenberg uncertainty principle.
Quantum teleportation is known as an important element in the quantum encryption. Quantum teleportation is a technique of moving only the quantum information of particles from one place to another. This quantum teleportation is realized when photons exchange information with each other, using photon pairs that have quantum correlation to each other (quantum correlation photon pairs). A quantum correlation photon pair has the property that quantum state of one photon determines that of the other, and this property does not depend on a distance between two photons.
Generation of quantum correlation photon pairs has been conventionally realized by parametric down conversion in which a nonlinear optical material is irradiated by high output laser. However, this has disadvantages in that (i) wavelengths of the quantum correlation photon pairs generated through this process are relatively long and (ii) it is difficult to realize a quantum correlation multi-photon state of more than three photons. In contrast, the following Non-Patent Document 1 and Patent Document 2 disclose a technique of generating correlation photon pairs having short wavelengths by using a resonance hyper-parametric scattering process of a semiconductor CuCl bulk crystal.
The generation method of the correlation photon pairs with use of the semiconductor CuCl bulk crystal has the most fundamental feature and advantages in that it is possible to generate the correlation photon pairs having shorter wavelengths, as compared to the conventional generation method. From perspective of realizing a device, however, it is more preferable to adopt a nano crystal than to adopt a bulk crystal. Interaction with light, nevertheless, will become small when the nano crystal is adopted. This causes a problem that it is difficult to generate correlation photon pairs with high efficiency.
Non-Patent Document 1: K. Edamatsu, G. Oohata, R. Shimizu, and T. Itoh: Nature, 431, 167, (2004).
Patent Document 2: Japanese Unexamined Publication of Patent Application, Tokukai 2005-309012 (published on Nov. 4, 2005).